Evaporation control using covers - some realistic solutions for the irrigation industry

Evaporation control using covers - some realistic solutions for the irrigation industry

[Abstract]: In Australia, up to 40% of the water stored in small farm dams over the summer months may be lost to evaporation. Reducing this loss using either physical or chemical covers therefore has the potential to significantly increase agricultural water use efficiency in Australia and therefore generate millions of dollars for the Australian economy.

A research project took place from 2003-2005 to investigate the performance of various different types of cover. Outcomes from the Queensland Natural Resources and Mines commissioned project included important new knowledge regarding the field performance of various different types of cover, technology to accurately assess evaporation and seepage losses and increased public awareness of the potential for evaporation reduction on water storages. Significant interest has been shown by landholders, agencies and consultancy companies in developing this further.

The project involved the development of a novel experimental method to measure and record water depth to an accuracy of ±1mm using highly accurate Pressure Sensitive Transducers (Craig et al 2005). PST units were placed at a constant 30cm height above the dam floor by a float-weight mechanism. The units were connected to dataloggers and the data was compared to weather station derived Penman-Monteith based estimates of evaporation. This enabled evaporation losses to be separated from seepage, also a very significant loss of Australian farm water.

The PST methodology confirmed that evaporation losses in small farm dams was typically 4-7mm/day in summer rising to 10mm/day when air temperatures exceeded 40° C. As a matter of interest, the analysis also revealed that Australian summer night-time evaporation due to heat advection effects could be as much as 10-20% of the total daily evaporation.

The PST analysis technique applied to covered dams revealed evaporation reduction performance figures of approximately 75% for shadecloth covered dams, up to 95% with dams covered with a properly functioning floating cover, and approximately 15 to 30% with dams covered with a cetyl alcohol based chemical monolayer (Barnes, 1993). Optimising the performance of monolayers with improved application technology is currently the focus of evaporation research efforts at NCEA.

The study has revealed that high evaporation savings are possible if physical covers are used on small farm dams less than 10ha in size. Physical covers can still be used on dams larger than 10ha in size, but may be prove economically impractical due to the high amounts of capital investment required. In the future, however, increasing cost of water may allow increasingly large sizes of dam to be covered. For the present, economic analyses have implied that chemical covers may represent the best option for evaporation control on large farm water storages.

Future research is planned commencing in June 2006 with the support of the CRC for Irrigation Futures and collaboration with CRC for Polymers and Nylex Water Solutions, producers of WaterSavr, the cetyl alcohol based chemical monolayer. A new applicator for applying the monolayer has recently been developed which relies on a compressed air system for delivery of the powder just below the water surface. The instrument was successfully demonstrated for the first time in Australia at USQ Toowoomba in the presence of David Verlee (Vice President of Business Development, Water Savr Global Solutions Inc, Richmond, Canada and Greg Phillips, Northern Regional Manager, Nylex Corporation. The applicator has been trialed at Byrock (80km south of Bourke, NSW) by Brendon Mason, Nylex Water Solutions.

The research will focus upon developing computer assisted application technology which will relies upon visualisation of the chemical coverage on the water surface. Fundamental research will be carried out on the performance of chemical monolayers in terms of their spread and resistance to breakup, microbial and UV degradation. Preliminary investigations have indicated that the performance of cetyl alcohol based monolayers may be significantly enhanced with the addition of other chemicals, for example, poly vinyl stearate. PVS is a polymer with a comb-like structure which may enhance the resistance of the monolayer to wind stress (Barnes, 1993).

The NCEA evaporation research team comprises Director Erik Schmidt, Ian Craig, Andrew Green and Michael Scobie. The project was funded by the Queensland Department of Natural Resources and Mines (NRM) through the Rural Water Use Efficiency Initiative (RWUEI). The project depended upon the cooperation of product suppliers including NetPro (shadecloth), Nylex Water Solutions (WaterSavr), Evaporation Control Systems (E-VapCap), Raftex, Ciba Speciality Chemicals, and site owners Moonrocks (St George), Cubbie Station (Dirranbandi), Renato Andreatta (Stanthorpe) and Peak Downs Shire Council (Capella).

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